• Tony Krowiak's avatar
    s390/vfio-ap: introduce new mutex to control access to the KVM pointer · 21195eb0
    Tony Krowiak authored
    The vfio_ap device driver registers for notification when the pointer to
    the KVM object for a guest is set. Recall that the KVM lock (kvm->lock)
    mutex must be taken outside of the matrix_dev->lock mutex to prevent the
    reporting by lockdep of a circular locking dependency (a.k.a., a lockdep
    splat):
    
    * see commit 0cc00c8d ("Fix circular lockdep when setting/clearing
      crypto masks")
    
    * see commit 86956e70 ("replace open coded locks for
      VFIO_GROUP_NOTIFY_SET_KVM notification")
    
    With the introduction of support for hot plugging/unplugging AP devices
    passed through to a KVM guest, a new guests_lock mutex is introduced to
    ensure the proper locking order is maintained:
    
    struct ap_matrix_dev {
            ...
            struct mutex guests_lock;
           ...
    }
    
    The matrix_dev->guests_lock controls access to the matrix_mdev instances
    that hold the state for AP devices that have been passed through to a
    KVM guest. This lock must be held to control access to the KVM pointer
    (matrix_mdev->kvm) while the vfio_ap device driver is using it to
    plug/unplug AP devices passed through to the KVM guest.
    
    Keep in mind, the proper locking order must be maintained whenever
    dynamically updating a KVM guest's APCB to plug/unplug adapters, domains
    and control domains:
    
        1. matrix_dev->guests_lock: required to use the KVM pointer - stored in
           a struct ap_matrix_mdev instance - to update a KVM guest's APCB
    
        2. matrix_mdev->kvm->lock: required to update a guest's APCB
    
        3. matrix_dev->mdevs_lock: required to access data stored in a
           struct ap_matrix_mdev instance.
    Signed-off-by: default avatarTony Krowiak <akrowiak@linux.ibm.com>
    Reviewed-by: default avatarJason J. Herne <jjherne@linux.ibm.com>
    Signed-off-by: default avatarAlexander Gordeev <agordeev@linux.ibm.com>
    21195eb0
vfio_ap_private.h 4.76 KB